Abstract:

A scan line profile characteristic representing the distortion of a scan
line is detected. Dot image data undergoes the screen process using a
dither matrix. At this time, the quantization process is done by shifting
a dither matrix element in the sub-scanning direction opposite to the
direction of the scan line changing process at a scan line changing point
in the scan line changing process in accordance with the profile
characteristic. The image data after the screen process undergoes the
scan line changing process, and the interpolation process smooths the
scan line changing point.

Claims:

1. An image forming apparatus which has, for each color component, image
forming means for forming an image, and forms a color image by
compositing images of respective color components, the apparatus
comprising:a screen processing unit configured to perform a screen
process for dot image data to be processed by shifting a position of a
dither matrix element in accordance with a shift amount of a scan line in
a sub-scanning direction on an image carrier of the image forming means;
anda registration error correction unit configured to shift, in the
sub-scanning direction, a position of each pixel of the dot image data
processed by said screen processing unit so as to cancel the shift amount
of the scan line in the sub-scanning direction on the image carrier of
the image forming means.

2. The apparatus according to claim 1, wherein said registration error
correction unit shifts, for each pixel in the sub-scanning direction, the
position of each pixel of dot image data to be processed so as to cancel
the shift amount of the scan line in the sub-scanning direction on the
image carrier of the image forming unit, and smooths a step of each
pixel.

3. The apparatus according to claim 1, wherein said screen processing
means comprisesa generation unit configured to generate a new dither
matrix by shifting a position of an element of an original dither matrix
in accordance with the shift amount of the scan line in the sub-scanning
direction on the image carrier of the image forming unit before
performing the screen process, anda unit configured to execute the screen
process by using the new dither matrix.

4. The apparatus according to claim 1, further comprising a rotation
processing unit configured to rotate the dot image data having undergone
the screen process,wherein said registration error correction unit
shifts, in the sub-scanning direction, the position of each pixel of dot
image data to be processed so as to cancel the shift amount of the scan
line in the sub-scanning direction after rotation by said rotation
processing unit.

5. The apparatus according to claim 4, wherein said screen processing unit
rotates the dither matrix in correspondence with a rotational angle by
said rotation processing unit, and performs the screen process using the
rotated dither matrix.

6. An image correction method in an image forming apparatus which has, for
each color component, image forming means for forming an image, and forms
a color image by compositing images of respective color components, the
method comprising:a screen processing step of performing a screen process
for dot image data to be processed by shifting a position of a dither
matrix element in accordance with a shift amount of a scan line in a
sub-scanning direction on an image carrier of the image forming means;
anda registration error correction step of shifting, in the sub-scanning
direction, a position of each pixel of the dot image data processed in
the screen processing step so as to cancel the shift amount of the scan
line in the sub-scanning direction on the image carrier of the image
forming means.

7. A computer-readable storage medium storing a program which causes an
image forming apparatus that has, for each color component, image forming
means for forming an image, and forms a color image by compositing images
of respective color components, to function asscreen processing means for
performing a screen process for dot image data to be processed by
shifting a position of a dither matrix element in accordance with a shift
amount of a scan line in a sub-scanning direction on an image carrier of
the image forming means, andregistration error correction means for
shifting, in the sub-scanning direction, a position of each pixel of the
dot image data processed by said screen processing means so as to cancel
the shift amount of the scan line in the sub-scanning direction on the
image carrier of the image forming means.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to an image forming apparatus and
image forming method and, more particularly, to an image forming
apparatus and image correction method for reproducing an input image at a
density for a stable quality in a laser beam printer (LBP), digital
copying machine, or multifunction printer (MFP) using an
electrophotographic process.

[0003]2. Description of the Related Art

[0004]As a kind of color image forming apparatus such as a printer or
copying machine, there is known a tandem type color image forming
apparatus, which comprises electrophotographic image forming units equal
in number to color components and sequentially transfers toner images of
respective color components onto a print medium by the image forming
units. The image forming unit of each color includes a developing unit
and photosensitive drum. It is known that the tandem type color image
forming apparatus has a plurality of factors which cause a positional
error (to be referred to as a registration error) between images of
respective color components.

[0005]These factors include the unevenness and attaching positional error
of the lens of a deflecting scanning unit including the optical system of
a polygon mirror, fθ lens, and the like, and the mounting
positional error of the deflecting scanning unit to the image forming
apparatus main body. Owing to these positional errors, the scan line does
not become a straight line parallel to the rotating shaft of the
photosensitive drum, and inclines or skews. If the degree of inclination
or skew of the scan line (to be referred to as the profile or shape of
the scan line hereinafter) is different between colors, a registration
error occurs.

[0006]The profile has different characteristics for respective image
forming apparatuses, that is, printing engines, and for deflecting
scanning units of respective colors. FIGS. 24A to 24D show examples of
the profile. In FIGS. 24A to 24D, the abscissa axis represents a position
in the main scanning direction in the image forming apparatus. A line
2411 expressed as a straight line in the main scanning direction
represents the characteristic (profile) of an ideal scan line free from a
skew. Curves 2401, 2402, 2403, and 2404 represent the profiles of
respective colors, and show examples of the profiles of scan lines for
cyan (to be referred to as C hereafter), magenta (to be referred to as M
hereafter), yellow (to be referred to as Y hereafter), and black (to be
referred to as K hereafter), respectively. The ordinate axis represents a
shift amount in the sub-scanning direction from an ideal characteristic.
As is apparent from FIGS. 24A to 24D, the curve of the profile is
different between colors. When electrostatic latent images are formed on
the photosensitive drums of image forming units corresponding to the
respective colors, the profile difference appears as the registration
error between image data of the respective colors.

[0007]As a measure against registration error, Japanese Patent Laid-Open
No. 2002-116394 discloses a method of measuring the degree of skew of a
scan line using an optical sensor in the process of assembling a
deflecting scanning device, mechanically rotating the lens to adjust the
skew of the scan line, and fixing the lens with an adhesive.

[0008]Japanese Patent Laid-Open No. 2003-241131 discloses a method of
measuring the inclination of a scan line using an optical sensor in the
process of mounting a deflecting scanning device into a color image
forming apparatus main body, mechanically inclining the deflecting
scanning device to adjust the inclination of the scan line, and then
mounting the deflecting scanning device into the color image forming
apparatus main body.

[0009]Japanese Patent Laid-Open No. 2004-170755 discloses a method of
measuring the inclination and skew of a scan line using an optical
sensor, correcting bitmap image data to cancel them, and forming the
corrected image. That is, a shift of an actual scan line from an ideal
scan line which is a straight line parallel on the surface of the
photosensitive drum to the rotating shaft of the photosensitive drum is
canceled by shifting image data by the same amount in an opposite
direction. This method corrects image data, and thus does not require a
mechanical adjustment member or adjustment step in assembly. This method
can downsize a color image forming apparatus, and deal with registration
error at a lower cost than those by methods disclosed in Japanese Patent
Laid-Open Nos. 2002-116394 and 2003-241131. The electrical registration
error correction is divided into correction of one pixel and that of less
than one pixel. In correction of one pixel, pixels are shifted (offset)
one by one in the sub-scanning direction in accordance with the
inclination and skew correction amounts, as shown in FIGS. 25A to 25C. In
the following description, a position where the pixel is offset will be
called a scan line changing point, and the process to offset a pixel will
be called a scan line changing process. In FIG. 25A, P1 to P5 are scan
line changing points.

[0010]In FIG. 25A, a profile 2501 of a scan line is corrected. The profile
2501 may also be expressed by an array of the coordinate values of pixels
on a scan line, but in FIG. 25A, is expressed by approximate straight
lines divided for respective areas. The scan line changing point is a
position in the main scanning direction where the profile is scanned in
the main scanning direction and shifts by one pixel in the sub-scanning
direction. In FIG. 25A, P1 to P5 are scan line changing points. At a scan
line changing point serving as a boundary, dots after the scan line
changing point are shifted by one line in a direction opposite to the
shift of the profile in the sub-scanning direction. This process is
executed by paying attention to each line. FIG. 25B shows an example of
image data shifted in the sub-scanning direction at each scan line
changing point. In FIG. 25B, each hatched portion 2511 is one line before
the scan line changing process, that is, one line in original image data.
As a result of the scan line changing process, each line shifts in a
direction in which the shift of the profile in the sub-scanning direction
is canceled. FIG. 25C shows an example of image data obtained in this
manner. Each hatched portion is one line before correction. In image
formation, corrected image data is formed for each line. For example,
normal image formation proceeds in the order of a line 2521, line 2522, .
. . . After image formation, a hatched portion which forms one line in
image data before correction is formed on an ideal scan line which should
be originally formed. However, the scan line changing process is done for
each pixel, so a shift of less than one pixel still remains in the
sub-scanning direction.

[0011]A shift of less than one pixel that cannot be completely corrected
by the scan line changing process is corrected by adjusting the tone
value of bitmap image data by preceding and succeeding pixels in the
sub-scanning direction, as exemplified in FIGS. 26A to 26F. More
specifically, when the characteristic represents an upward inclination,
like a profile 2601 in FIG. 26A, bitmap image data before tone correction
is corrected to a pixel array 2603 (shown in FIG. 26C) inclined in a
direction (downward in this example) opposite to the inclination of the
profile. FIG. 26B shows bitmap image data before correction. Image data
2602 is shifted by one pixel in the sub-scanning direction at scan line
changing points P1 and P2, as shown in FIG. 26F. To make the image data
2602 close to the ideal image data 2603 after correction, tone correction
is executed to smooth steps at the scan line changing points P1 and P2,
as shown in FIG. 26D. FIG. 26D is a view schematically showing the
densities of pixels by the width and intensity of a laser pulse for
forming these pixels. After exposure, a latent image as shown in FIG. 26E
is formed to smooth steps generated by the scan line changing process.
According to this method, the image process can correct the registration
error. Tone correction performed for smoothing after the scan line
changing process will be called an interpolation process.

[0012]When the bitmap image remains as a halftone image, registration
error correction can be done by this sequence in accordance with the
profile of the image forming unit. However, the screen process sometimes
degrades the image quality.

[0013]FIGS. 10A to 10C are views schematically showing a state in which
the scan line changing process and interpolation process are performed
for a halftone image reproduced by the screen process. Binary image data
having undergone the screen process has a dot pattern (called a dither
pattern) corresponding to the tone level owing to the locality meaning
that pixels in a very small area have similar tone levels. The dot
pattern is determined by the arrangement of the threshold matrix of a
dither matrix. In some cases, the dot pattern is designed to have screen
angles different between, for example, color components. In this example,
binary image data after the screen process is expressed by four bits per
pixel. That is, the pixel value after the screen process is 0 or 15.

[0014]If the scan line changing process is done for image data having
undergone the screen process, the dither pattern of an output image
shifts at a scan line changing point. For example, when an image 1001
shown in FIG. 10A is input, dots shift before and after a scan line
changing point, as shown in FIG. 10B. As a result, the dither pattern
shifts at the scan line changing point serving as a boundary. This shift
is observed as a stripe running in the sub-scanning direction. This
stripe degrades the image quality.

[0015]If the above-mentioned interpolation process is applied to image
data after the screen process in addition to the scan line changing
process, areas before and after the scan line changing point are
reproduced at a density different from that of a peripheral area,
generating density unevenness as shown in FIG. 10C.

[0016]If the screen process is performed using a dither matrix for image
data after the scan line changing process, no dither pattern shifts and
no image quality degrades. However, the scan line changing process
requires a large-capacity memory. In order to execute the scan line
changing process for unquantized image data without performing the screen
process, line buffers equal in number to lines subjected to the scan line
changing process are necessary. In addition, each pixel has a size before
quantization. For this reason, a large-capacity memory is required.

SUMMARY OF THE INVENTION

[0017]The present invention has been made in consideration of the
conventional situation, and has as its object to solve the
above-described problems. More specifically, it is an object of the
present invention to provide an image forming apparatus and image
correction method capable of correcting, by the scan line changing
process, a registration error caused by the profile difference between
image forming units of respective color components, and preventing
degradation of the image quality caused by a shift of the dither pattern,
thereby obtaining a high-quality image with a small circuit arrangement.

[0018]It is another object of the present invention to provide an image
forming apparatus and image correction method capable of preventing
degradation of the image quality even if the rotation process is
performed after the registration error correction process and screen
process.

[0019]It is still another object of the present invention to provide an
image forming apparatus and image correction method capable of preventing
degradation of the image quality caused by a change of the screen angle
upon rotation of an image.

[0020]To achieve the above objects, the present invention comprises the
following arrangement. That is, an image forming apparatus which has, for
each color component, image forming means for forming an image, and forms
a color image by compositing images of respective color components, the
apparatus comprises:

[0021]a screen processing unit configured to perform a screen process for
dot image data to be processed by shifting a position of a dither matrix
element in accordance with a shift amount of a scan line in a
sub-scanning direction on an image carrier of the image forming means;
and

[0022]a registration error correction unit configured to shift, in the
sub-scanning direction, a position of each pixel of the dot image data
processed by the screen processing unit so as to cancel the shift amount
of the scan line in the sub-scanning direction on the image carrier of
the image forming means.

[0023]According to another aspect of the present invention, an image
correction method in an image forming apparatus which has, for each color
component, image forming means for forming an image, and forms a color
image by compositing images of respective color components, the method
comprises:

[0024]a screen processing step of performing a screen process for dot
image data to be processed by shifting a position of a dither matrix
element in accordance with a shift amount of a scan line in a
sub-scanning direction on an image carrier of the image forming means;
and

[0025]a registration error correction step of shifting, in the
sub-scanning direction, a position of each pixel of the dot image data
processed in the screen processing step so as to cancel the shift amount
of the scan line in the sub-scanning direction on the image carrier of
the image forming means.

[0026]The present invention can correct a registration error caused by the
profile difference between image forming units of respective color
components, and prevent degradation of an image caused by correction,
thereby obtaining a high-quality image with a small circuit arrangement.

[0027]Further features of the present invention will become apparent from
the following description of exemplary embodiments (with reference to the
attached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a flowchart showing the process of an image processing
apparatus according to the present invention;

[0030]FIGS. 3A and 3B are graphs showing the profile characteristics of a
scan line;

[0031]FIG. 4 is a block diagram of the arrangement of the color image
forming apparatus;

[0032]FIGS. 5A to 5G are views showing an interpolation method at a scan
line changing point;

[0033]FIGS. 6A to 6D are views showing examples of the weighting
arrangement;

[0034]FIGS. 7A to 7D are graphs showing a direction in which correction
should be done, and the shift direction;

[0035]FIGS. 8A to 8C are views showing a registration error and a scan
line changing process;

[0036]FIGS. 9A to 9D are views showing how to hold data of the profile
characteristic;

[0037]FIGS. 10A to 10C are views showing a state in which the scan line
changing process and interpolation process are performed for a halftone
image based on a screen having undergone the scan line changing process;

[0038]FIGS. 11A to 11D are views showing a state in which the screen
process and phase offset process are performed for an input image;

[0039]FIG. 12 is a view showing an example of the array of dither matrices
shifted in the sub-scanning direction every several lines;

[0040]FIG. 13 is a flowchart of a screen process including a phase offset
process in the first embodiment;

[0041]FIG. 14 is a view showing the relationship between an input image
and a dither matrix;

[0042]FIG. 15 is a view showing a state in which dither tables are
periodically arrayed;

[0043]FIGS. 16A to 16G are views showing an intermediate image and output
result in a case where an image process according to the embodiment is
performed for an input image, and those in a case where it is not
performed;

[0044]FIGS. 17A and 17B are views showing the array of dither matrices
shifted in the main scanning direction;

[0045]FIGS. 18A and 18B are views showing the array of dither matrices of
a shape other than the square or rectangle;

[0046]FIG. 19 is a flowchart showing a screen process including a phase
offset process in the second embodiment;

[0047]FIGS. 20A and 20B are views showing an output image which is not
rotated in an image forming apparatus, and an output image which is
rotated;

[0048]FIG. 21 is a view showing the relationship between X, Y, X_MAX,
Y_MAX, Xn, and Yn;

[0049]FIGS. 22A to 22C are views showing an unrotated output image, a
rotated output image, and an intermediate image when rotating an output
image in the fourth embodiment;

[0050]FIG. 23 is a view showing the relationship between X1, Y1, X_DMAX,
Y_DMAX, X1n, and Y1n;

[0051]FIGS. 24A to 24D are graphs showing examples of the profile
characteristic;

[0052]FIGS. 25A to 25C are views showing a scan line changing process; and

[0053]FIGS. 26A to 26F are views showing an interpolation process.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

[0054]The first embodiment of the present invention will be described with
reference to the accompanying drawings. In the first embodiment, a shift
of an actual scan line from an ideal scan line which should be originally
formed by scanning the surface of a photosensitive drum with a laser
beam, that is, from a scan line parallel to the rotating shaft of the
photosensitive drum is canceled by shifting dot image data by the same
amount in an opposite direction. Image degradation such as unevenness
generated upon registration error correction is prevented. In addition,
image degradation caused by performing a dither process for dot image
data after registration error correction is also prevented.

[0055]An example of the structure of a laser beam printer as an example of
an image forming apparatus applicable as an embodiment of the present
invention, and an image correction method executed by the laser printer
will be explained. The embodiment is applicable not only to the laser
beam printer, but also to another type of output apparatus such as an
inkjet printer or MFP (Multi Function Printer/Multi Function Peripheral).
However, a printer to which the present invention can be effectively
applied is one which comprises image forming units for respective color
components and therefore may suffer a registration error between images
of the respective color components. The registration error may occur when
the inkjet printer is a serial printer in which printheads for respective
color components are mounted in independent carriages, or a line head
printer in which printheads for respective color components are
independently attachable. By applying the embodiment of the present
invention to these printers, the image quality improves. However, a
tandem type color laser printer is highly likely to have a difference in
scan line profile between color components, so the embodiment will
exemplify the tandem type color laser printer.

Image Forming Section of Tandem Color LBP

[0056]FIG. 4 is a block diagram for explaining the arrangements of blocks
associated with formation of an electrostatic latent image in an
electrophotographic color image forming apparatus according to the first
embodiment. The color image forming apparatus comprises a color image
forming section 401 and image processing section 402. The image
processing section 402 generates bitmap image information, and the color
image forming section 401 forms an image on a print medium based on the
bitmap image information. The image processing section 402 also performs
a correction process such as registration error correction by referring
to pieces of profile information 416C, 416M, 416Y, and 416K which are
measured in advance and stored in a profile storage unit 403 for image
forming units of respective color components. In the following
description, building components denoted by reference numerals with color
symbols "C", "M", "Y", and "K" for respective color components are
sometimes generically named by reference numerals without these color
symbols. The image forming unit is the name of a unit which includes a
scanner unit 414 and printing unit 415 and forms a single-color image for
each color component. The printing unit 415 is a unit which includes a
photosensitive drum, transfer drum, and the like and forms a toner image.
The printing unit 415 also forms images in addition to characters.

[0057]FIG. 2 is a sectional view of the tandem type color image forming
section 401 adopting an intermediate transfer member 28 as an example of
the electrophotographic color image forming apparatus. The operation of
the color image forming section 401 in the electrophotographic color
image forming apparatus will be explained with reference to FIG. 2. The
color image forming section 401 drives exposure light in accordance with
an exposure time processed by the image processing section 402, forming
an electrostatic latent image on the photosensitive drum, that is, image
carrier. The color image forming section 401 develops the electrostatic
latent image to form a single-color toner image of each color component.
The color image forming section 401 composites single-color toner images
on the intermediate transfer member 28 to form a multi-color toner image.
The color image forming section 401 transfers the multi-color toner image
to a print medium 11, and thermally fixes it. The intermediate transfer
member also serves as an image carrier. The charging means comprises four
injection chargers 23Y, 23M, 23C, and 23K for charging photosensitive
bodies 22Y, 22M, 22C, and 22K for Y, M, C, and K. The injection chargers
incorporate sleeves 23YS, 23MS, 23CS, and 23KS.

[0058]Driving motors rotate the image carriers, that is, photosensitive
bodies (photosensitive drums) 22Y, 22M, 22C, and 22K counterclockwise in
accordance with the image forming operation. The scanner units 414Y,
414M, 414C, and 414K serving as exposure means irradiate the
photosensitive bodies 22Y, 22M, 22C, and 22K with exposure light,
selectively exposing the surfaces of the photosensitive bodies 22Y, 22M,
22C, and 22K. As a result, electrostatic latent images are formed on the
surfaces of the photosensitive bodies. Developing units 26Y, 26M, 26C,
and 26K serving as developing means develop the electrostatic latent
images with Y, M, C, and K toners supplied from toner cartridge 25Y, 25M,
25C, and 25K in order to visualize the electrostatic latent images. The
developing units incorporate sleeves 26YS, 26MS, 26CS, and 26KS. Each
developing unit 26 is detachable. Each scanner unit can express the tone
of each pixel, for example, 16 tone levels in accordance with the width
and intensity of a laser beam.

[0059]Primary transfer rollers 27Y, 27M, 27C, and 27K serving as transfer
means press the intermediate transfer member 28 rotating clockwise
against the photosensitive bodies 22Y, 22M, 22C, and 22K, transferring
the toner images on the photosensitive bodies to the intermediate
transfer member 28. A single-color toner image is efficiently transferred
onto the intermediate transfer member 28 by applying a proper bias
voltage to the primary transfer roller 27, and making the rotational
speed of the photosensitive body 22 different from that of the
intermediate transfer member 28. This transfer is called primary
transfer.

[0060]A multi-color toner image obtained by compositing single-color toner
images of stations (which mean the image forming units of the respective
color components) is conveyed to a secondary transfer roller 29 as the
intermediate transfer member 28 rotates. The multi-color toner image on
the intermediate transfer member 28 is transferred onto the print medium
11 which is conveyed from a paper feed tray 21a and 21b to the secondary
transfer roller 29 while being clamped. A proper bias voltage is applied
to the secondary transfer roller 29 to electrostatically transfer the
toner image. This transfer is called secondary transfer. While
transferring the multi-color toner image onto the print medium 11, the
secondary transfer roller 29 abuts against the print medium 11 at a
position 29a, and moves apart from the print medium 11 to a position 29b
after printing.

[0061]A fixing unit 31 comprises a fixing roller 32 for heating the print
medium 11, and a press roller 33 for pressing the print medium 11 against
the fixing roller 32, in order to fuse and fix, on the print medium 11, a
multi-color toner image transferred on the print medium 11. The fixing
roller 32 and press roller 33 are hollow and incorporate heaters 34 and
35, respectively. The fixing unit 31 conveys the print medium 11 bearing
the multi-color toner image by the fixing roller 32 and press roller 33,
and applies heat and a pressure to fix the toner to the print medium 11.

[0062]The toner-fixed print medium 11 is discharged by discharge rollers
(not shown) onto a delivery tray (not shown), ending the image forming
operation. A cleaning unit 30 cleans off toner left on the intermediate
transfer member 28. Waste toner left after transferring four color toner
images formed on the intermediate transfer member 28 to the print medium
11 is stored in a cleaner vessel. As described above, the tandem color
LBP comprises the image forming units including the printing units 415
and scanner units 414 for the respective color components. In FIG. 2,
regarding the scanner units 414, scanner units 414Y, 414M, 414C and 414K
are shown for respective color components. Regarding the printing units
415, only printing unit 415Y for yellow is exemplarily illustrated.

Profile Characteristic of Scan Line

[0063]The profile characteristic of an actual scan line 302 for each color
in the image forming apparatus will be explained with reference to FIGS.
3A and 3B. In FIGS. 3A and 3B, the scan line 302 represents an actual
scan line which inclines or skews owing to the positional precision and
eccentricity of the photosensitive body 22, and the positional precisions
of the optical systems in the scanner units 414, that is, 414C, 414M,
414Y, and 414K shown in FIG. 2. The image forming apparatus has a
different profile characteristic represented by the scan line 302 for
each printing device (printing engine). The scan line 302 is frequently
referred to as a profile 302 hereinafter. In a color image forming
apparatus, the profile characteristic is different between colors.

[0064]FIG. 3A is a graph showing part of the profile characteristic of the
image forming apparatus, and shows an area where the profile
characteristic shifts upward in the sub-scanning direction. FIG. 3B shows
an area where the profile characteristic shifts downward in the
sub-scanning direction. An abscissa axis 301 represents an ideal scan
line, and shows a characteristic when the photosensitive body 22 is
scanned perpendicularly to the rotational direction of the photosensitive
body 22, that is, scanned parallel to the rotating shaft. The profile is
expressed by a graph in FIGS. 3A and 3B, but a profile held in the
profile information 416 is discrete data. For example, every time an
actual scan line moves apart from or close to an ideal scan line by one
pixel from a scan line start position P0, the position and the moving
direction representing whether the actual scan line moves apart from or
close to an ideal scan line are stored in association with each other.
The position suffices to specify the ordinal number of a pixel in the
scan line direction. Hence, the profile 302 is approximated by line
segments 311, 312, 313, and 314 in profile information, which is
sufficient for registration error correction.

[0065]In the following description, the profile characteristic assumes a
direction in which the image processing section 402 corrects the profile
characteristic. However, this representation is merely an example, and
any representation can be adopted as long as the shift amount and
direction can be uniquely specified. For example, it is possible to
define the profile characteristic as the shift direction in the color
image forming section 401 and correct the characteristic in the opposite
direction by the image processing section 402.

[0066]FIGS. 7A to 7D show the correlation between the direction in which
the image processing section 402 performs correction, and the shift
direction of the scan line in the color image forming section 401 on the
basis of the profile definition. When the profile characteristic of the
color image forming section 401 is given as shown in FIG. 7A, the image
processing section 402 shifts image data in an opposite direction in the
sub-scanning direction, as shown in FIG. 7B. When the profile
characteristic of the color image forming section 401 is given as shown
in FIG. 7C, the image processing section 402 shifts image data in the
sub-scanning direction, as shown in FIG. 7D. Note that the shift amount
is measured using the ideal scan line 301 as a reference.

[0067]Profile characteristic data (profile information) includes the pixel
position of a scan line changing point in the main scanning direction,
and the direction of change of the scan line to the next scan line
changing point, as shown in FIG. 9B. More specifically, scan line
changing points P1, P2, P3, . . . , Pm are defined for the profile
characteristic in FIG. 9A. Each scan line changing point is defined as a
point where the scan line shifts by one pixel in the sub-scanning
direction. As the direction, the scan line shifts upward or downward in a
section till the next scan line changing point. For example, at the scan
line changing point P2, the scan line shifts upward by one line in FIG.
9A. That is, at the scan line changing point P2, image data changes to a
line immediately below the current line. The shift direction at the point
P2 is "upward (↑)", as shown in FIG. 9B. In the image process,
image data changes to a lower line. Similarly at the point P3, the shift
direction is "upward (↑)". The shift direction in the sub-scanning
direction at the scan line changing point P4 is "downward (↓)",
unlike the preceding direction. Data on the direction is held as, for
example, "1"representing the upward direction, or "0" representing the
downward direction, as shown in FIG. 9C. In this case, the amount of held
data corresponds to bits equal in number to scan line changing points. If
the number of scan line changing points is m, the number of held bits is
also m. Further, a bit string representing shifted lines may also be held
as shown in FIG. 9D, instead of holding the positions of scan line
changing points. FIG. 9D shows a phase offset table (to be described
later), and shows the cumulative number of shifted lines (one line in
this example) in the shift direction at each scan line changing point. An
upward shift of the profile in FIG. 9A is given by a positive value, a
downward shift is given by a negative value, and these values are added.
That is, FIG. 9D shows the relative line number of a line to which the
line of interest changes in the scan line changing process when the input
line number is 0. In FIG. 9D, the sign is opposite to that of the scan
line changing process, and is the same as that of the profile
characteristic.

Scan Line Changing Point

[0068]The scan line changing point of an area where the scan line shifts
upward in the laser scanning direction will be explained with reference
to FIG. 3A. The scan line changing point in the embodiment is a point
where the scan line shifts by one pixel in the sub-scanning direction. In
FIG. 3A, points P1, P2, and P3 where the upward skew characteristic 302
shifts by one pixel in the sub-scanning direction are scan line changing
points. In FIG. 3A, the points P1, P2, and P3 are plotted using P0 as a
reference. As is apparent from FIG. 3A, the distance between scan line
changing points is short in an area where the skew characteristic 302
changes abruptly, and long in an area where it changes gradually, as
represented by distances L1 and L2.

[0069]The scan line changing point of an area where the scan line shifts
downward in the laser scanning direction will be explained with reference
to FIG. 3B. Also in an area representing a downwardly shifted
characteristic, the scan line changing point is defined as a point where
the scan line shifts by one pixel in the sub-scanning direction. In FIG.
3B, points Pn and Pn+1 where the downward skew characteristic 302 shifts
by one pixel in the sub-scanning direction are scan line changing points.
Also in FIG. 3B, similar to FIG. 3A, the distance between scan line
changing points is short in an area where the skew characteristic 302
changes abruptly, and long in an area where it changes gradually, as
represented by distances Ln and Ln+1.

[0070]As described above, the scan line changing point is closely related
to the degree of change of the skew characteristic 302 of the image
forming apparatus. The number of scan line changing points is large in an
image forming apparatus having a steep skew characteristic, and small in
an image forming apparatus having a gradual skew characteristic.

[0071]If the skew characteristic of the image forming unit is different
between colors, the number and positions of scan line changing points are
also different. The difference in scan line profile between colors
appears as a registration error in an image obtained by transferring
toner images of all colors onto the intermediate transfer member 28. The
present invention is directed to a process at the scan line changing
point.

Image Processing Section of Tandem Color LBP

[0072]The image processing section 402 in the color image forming
apparatus will be explained with reference to FIGS. 1 and 4. FIG. 1 shows
an outline of the process. First, profile characteristic information is
detected (or stored profile characteristic information is read out)
(S101), and the dither process (screen process) is performed using a
phase offset table corresponding to the profile characteristic
information (S102). Then, the scan line changing process (S103) and the
interpolation process (S104) are performed. The processed dot image data
is transmitted to the color image forming section and printed. Details of
this process will be explained below.

[0073]An image generation unit 404 generates raster image data capable of
a printing process from print data received from a computer or the like
(not shown), and outputs the raster image data for each pixel as R, G,
and B data and attribute data representing the data attribute of each
pixel. The image generation unit 404 may also be configured to arrange a
reading means in the color image forming apparatus and process image data
from the reading means instead of image data received from a computer or
the like. A color conversion unit 405 converts R, G, and B data into C,
M, Y, and K data in accordance with the toner colors of the color image
forming section 401, and stores the C, M, Y, and K data and attribute
data in a storage unit 406. The storage unit 406 is the first storage
unit arranged in the image processing section 402, and temporarily stores
dot image data subjected to a printing process. The storage unit 406 may
also be formed from a page memory which stores dot image data of one
page, or a band memory which stores data of lines. Dot image data is also
called raster image data.

[0074]Halftone processing units 407C, 407M, 407Y, and 407K perform a
halftone process for attribute data and data of the respective colors
output from the storage unit 406. As concrete arrangements of the
halftone processing unit, there are a halftone processing unit which
performs a screen process, and a halftone processing unit which performs
an error diffusion process. The screen process is to perform an N-ary
process using predetermined dither matrices and input image data. The
error diffusion process is to perform an N-ary process by comparing input
image data with a predetermined threshold, and diffuse the difference
between the input image data and the threshold to peripheral pixels
subjected to the N-ary process later. The first embodiment executes the
error diffusion process. In the first embodiment, N=2, but the number of
bits per pixel is four. That is, a pixel value is converted into 0 or 15
by a quantization process.

[0075]A second storage unit 408 is incorporated in the image forming
apparatus, and stores N-ary data processed by the halftone processing
units 407, that is, 407C, 407M, 407Y, and 407K. If the position of a
pixel subjected to an image process by processing blocks on the
downstream side of the second storage unit 408 is a scan line changing
point, scan line changing of one line is executed when reading out data
from the second storage unit 408. More specifically, the address of a dot
to be read out proceeds not to the next dot but further by one line from
the next dot, or returns by one line. Whether to proceed or return the
address by one line is determined in accordance with the shift direction.

[0076]FIG. 8A is a view schematically showing the state of data held in
the storage unit 408 of FIG. 4. As shown in FIG. 8A, the storage unit 408
stores data processed by the halftone processing unit 407 regardless of
the correction direction of the image processing section 402 or the skew
characteristic of the scan line in the color image forming section 401.
If the direction in which the image processing section 402 performs
correction is downward, that is, the profile characteristic is downward,
image data is shifted upward by one pixel at a scan line changing point
serving as a boundary, as shown in FIG. 8B, when reading out a line 701
in FIG. 8A. If the direction in which the image processing section 402
performs correction is upward, that is, the profile characteristic is
upward, image data is shifted downward by one pixel at a scan line
changing point serving as a boundary, as shown in FIG. 8C, when reading
out image data of the line 701 from the storage unit 408.

[0077]Interpolation determining units 409C, 409M, 409Y, and 409K for the
respective colors determine whether or not the pixel requires
interpolation later as a process for pixels before and after a scan line
changing point in input N-ary data. Timing adjusting units 410C, 410M,
410Y, and 410K synchronize N-ary data read out from the storage unit 408
with the determination results of the interpolation determining units
409. Transfer buffers 411C, 411M, 411Y, and 411K temporarily hold data
output from the interpolation determining units 409 and timing adjusting
units 410. In this description, the first storage unit 406, second
storage unit 408, and transfer buffer 411 are separately arranged, but a
common storage unit may also be arranged in the image forming apparatus.

[0078]Interpolation processing units 412C, 412M, 412Y, and 412K
interpolate data received from the transfer buffers 411 based on the
determination results of the interpolation determining units 409 that are
also transferred from the transfer buffers. Although the determination
result from the interpolation determining unit 409 is the result of
determination of each pixel, the interpolation process by the
interpolation processing unit 412 uses pixels before and after a scan
line changing point corresponding to the profile (skew characteristic) of
the image forming apparatus. FIGS. 5A to 5G show an interpolation method
at a scan line changing point (FIGS. 5A to 5G will be referred to as FIG.
5 at once).

Interpolation Process

[0079]FIG. 5A is a graph showing the skew characteristic of the scan line
of the image forming apparatus in the laser scanning direction. Area 1 is
an area where the image processing section 402 needs to perform
correction downward. To the contrary, area 2 is an area where the image
processing section 402 needs to perform correction upward. For
descriptive convenience, the minimum interval between scan line changing
points is 16 pixels in the following description of the interpolation
process, but the present invention is not limited to this. The interval
may also be set to an arbitrary number of pixels, or the power of two in
order to reduce the circuit arrangement. Interpolation, that is,
smoothing to be described later is done for 16 pixels immediately before
a scan line changing point in the main scanning direction. If the
interval between scan line changing points is longer than 16 pixels,
pixels preceding to (on the left side in FIG. 5A) the smoothed area
remain without being smoothed. The interval is set to 16 pixels because
one binary pixel is represented by four bits in this example and can also
be represented by 16 tone levels in accordance with the tone expression
capability of the image forming unit. A step between lines can be
smoothed by changing the density by one tone level for one pixel value.

[0080]FIG. 5B shows images before and after a scan line changing point Pc
before the scan line changing process, that is, shows output image data
502 from the halftone processing unit 407 in the example of FIGS. 5A to
5G. The line of interest is the center line of 3-line image data shown in
FIG. 5B. FIG. 5C shows the arrangement of data 503 after the scan line
changing process of one pixel when paying attention to the line of
interest, that is, the arrangement of image data output from the storage
unit 408. Since the scan line changing process is performed when reading
out image data from the storage unit 408, the arrangement of pixels
before and after the scan line changing point Pc when inputting image
data to the interpolation processing unit 412 has a step of one line at
the scan line changing point Pc serving as a boundary.

[0081]The interpolation processing unit 412 executes the interpolation
process for image data appearing as a step on the line of interest. Since
the correction direction in area 1 is upward, the line of interest is
interpolated by weighting image data of a succeeding line. Weighting in
this description is to adjust the sum of two target pixels in the
sub-scanning direction to 16 in accordance with the minimum value of the
scan line changing point, as shown in FIG. 5C. However, this is merely an
example, and the sum of pixel values is not limited to 16. The sum of
pixel values may also be set to the power of two in order to reduce the
circuit used for calculation, or an arbitrary coefficient may also be
used for calculation in order to increase the precision. As the weighting
calculation, the weighting coefficient may also be changed for each
pixel, which will be described later. Alternatively, a common weighting
coefficient may also be used for a plurality of pixels, as shown in FIGS.
6A to 6D. Further, the number of corresponding pixels may also be changed
depending on the value of the weighting coefficient. The scan line
changing point is defined as a position on the main scan line where the
scan line shifts by one pixel in the sub-scanning direction. In the
following description, the reference position in interpolation is set to
the start point of main scanning, that is, the left end. Equation (1) is
used for interpolation, wherein x represents the position of the pixel of
interest in the main scanning direction, and y represents the position of
the pixel of interest in the sub-scanning direction. Letting p be a pixel
value and p' be a corrected pixel value, equation (1) is

p'(x,y)=w1×p(x,y-1)+w2×p(x,y)+w3×p(x, y+1) (1)

where w1, w2, and w3 are weighting coefficients having the same
x-coordinate and are defined by a coefficient matrix of 3×16 pixels
in this example, as shown in FIG. 5C. The coefficient matrix in FIG. 5C
is used to shift image data to an upper line at a scan line changing
point. All coefficients on a line immediately above the line of interest
are 0. The coefficient value on the line of interest (center line in FIG.
5C) is decremented by 1/16 from 15/16 to 0/16 (the denominator is not
shown in FIG. 5C) every time the pixel of interest moves to the right by
one pixel. The coefficient value on a line immediately below the line of
interest is incremented by 1/16 from 1/16 to 16/16 every time the pixel
of interest moves to the right by one pixel. This coefficient matrix
corresponds to 3×16 pixels centered on the line of interest
immediately before (on the right side) the scan line changing point, and
corrected pixel values are obtained in accordance with equation (1). The
corrected pixel values replace pixel values before correction. This
process is done by paying attention to all lines of image data to be
processed. Equation (1) represents the value of the pixel of interest by
the weighted average of the value of the pixel of interest and the values
of corresponding pixels on upper and lower lines.

[0082]FIG. 5D is a conceptual view of interpolated pixel values obtained
by applying equation (1) to image data in FIG. 5B. As for pixels before
the scan line changing point Pc, as the pixel is closer to the scan line
changing point Pc, it is more strongly influenced by a pixel value on a
succeeding line by the interpolation of equation (1). As the pixel (pixel
on the left side) is farther from the scan line changing point Pc, it is
more strongly influenced by the line of interest, that is, black data
line.

[0083]As for pixels after the scan line changing point Pc, as the pixel is
closer to the scan line changing point Pc, it is more strongly influenced
by image data on a line preceding to the line of interest. As the pixel
is farther from the scan line changing point Pc, it is more strongly
influenced by a line succeeding to the line of interest. The line
preceding to the line of interest is a previous line of interest which
becomes a preceding line of data owing to a scan line changing process
step larger than one pixel. In this example, pixels other than 16 pixels
immediately before the scan line changing point do not undergo the
interpolation process, so their image data are not smoothed.

[0084]Area 1 where correction needs to be performed downward will be
explained. When performing correction downward, weighting coefficients
used to calculate corrected pixel values are set on the line of interest
and a line preceding to it.

[0085]FIG. 5E shows image data output from the halftone processing unit
407. FIG. 5F shows an example of image data read out from the storage
unit 408. Since downward correction is done at a scan line changing point
Pa, a scan line changing process step larger than one pixel appears at
the scan line changing point Pa serving as a boundary, as shown in FIG.
5F. The values W1, W2, and W3 when performing downward correction are
those shown in FIG. 5F. For descriptive convenience, the sum of weighting
coefficients is set to 16, similar to the upward correction process. By
applying equation (1) to even downward correction, corrected pixel values
are obtained using the scan line changing point Pa as a boundary. Before
the scan line changing point Pa, as the pixel is closer to the scan line
changing point, it is more strongly influenced by a pixel value on a
preceding line. As the pixel is farther from the scan line changing point
Pa, it is more strongly influenced by the line of interest. As for pixels
after the scan line changing point Pa, as the pixel is closer to the scan
line changing point Pa, it is more strongly influenced by the line of
interest. As the pixel is farther from the scan line changing point Pa,
it is more strongly influenced by a line preceding to the line of
interest (FIG. 5G). In this example, the interpolation process targets 16
pixels before the scan line changing point. In FIG. 5G, the interval
between the scan line changing points Pa and Pb is 16 pixels, so image
data seem to be smoothed before and after the scan line changing point
Pa. However, when the interval is larger than 16 pixels, image data are
not smoothed immediately after the scan line changing point Pa.

[0086]In this way, a large step is prevented from appearing in pixel data
successive in the main scanning direction owing to a scan line changing
process step larger than one pixel in the interpolation process by the
interpolation processing unit 412 regardless of whether the correction
direction is upward or downward.

[0087]PWMs (Pulse Width Modulators) 413C, 413M, 413Y, and 413K convert
image data of the respective colors output from the interpolation
processing units 412C, 412M, 412Y, and 412K into the exposure times of
the scanner units 414C, 414M, 414Y, and 414K. The printing units 415C,
415M, 415Y, and 415K of the image forming section 401 output the
converted image data. Profile characteristic data are held in the image
forming section 401 as the characteristics of the image forming apparatus
(the profiles 416C, 416M, 416Y, and 416K). The image processing section
402 executes a scan line changing process and interpolation process in
accordance with the profile characteristics held in the image forming
section 401.

Screen Process

[0088]The most characteristic part of the present invention will be
described in more detail with reference to the accompanying drawings. As
described above, an electrophotographic image forming apparatus
reproduces an image by a halftone process such as a screen process.
However, if the registration error correction process, particularly the
scan line changing process is directly executed for a halftone image
having undergone the screen process, a phase mismatch of the dither
pattern occurs before and after a scan line changing point. To prevent
this, the halftone processing unit 407 executes a process (to be referred
to as a phase offset process hereinafter) to offset the phase of the
dither pattern in advance in a direction opposite to that of the scan
line changing process by referring to a scan line changing point set in
accordance with each of profile characteristics 416C, 416M, 416Y and
416K.

[0089]A screen process including the phase offset process by the halftone
processing unit 407 will be explained. FIGS. 11A to 11D schematically
show a state in which the halftone processing unit 407 performs the
screen process and phase offset process for an image input from the
storage unit 406. The phase offset process is unique to the embodiment. A
process to offset the dither matrix in advance so as to return the screen
to an original pattern by the scan line changing process when performing
the screen process prior to the scan line changing process will be called
the phase offset process.

[0090]The screen process will be explained first. FIG. 11A shows an image
1101 input from the storage unit 406 to the halftone processing unit 407.
Since an electrophotographic image forming apparatus is generally a
binary printer, the intermediate density is expressed by the area ratio
of output paper and toner in a region obtained by dividing an image into
small-area regions. This is so-called area coverage modulation. To obtain
the area of color in each region, a submatrix called a dither matrix
exemplified in FIG. 11C is prepared. The dither matrix has a threshold at
a portion corresponding to each pixel with the same shape and area as
those of a region serving as the unit of tone expression. For descriptive
convenience, one type of dither matrix is used, but the halftone
processing units 407C, 407M, 407Y, and 407K may also hold dither matrices
that are different between the respective colors. Dither matrices are
arrayed in a lattice, as shown in FIG. 11C, and superposed on an input
image. The pixel value of the input image is compared with the threshold
of the dither matrix for each pixel. It is determined from the magnitude
relation whether to color the target pixel. As a result, an image having
undergone the screen process as shown in FIG. 11D is obtained. In an
actual process, a pixel input in the raster scanning order is compared
with a threshold at a corresponding position in the dither matrix, and is
binarized. However, this process intuitively seems to be one shown in
FIG. 11C. In the following description, therefore, dot image data is
rasterized in this way, dither matrices are arrayed, and a pixel is
compared with a corresponding threshold and binarized. Note that the
dither matrix array pattern is not limited to a square lattice, and
includes the array of dither matrices staggered in the sub-scanning
direction every several lines, as shown in FIG. 12.

[0091]FIG. 13 is a flowchart of the screen process including the phase
offset process in the halftone processing unit 407. FIG. 14 is a
schematic view showing the relationship between an input image and a
dither matrix. (X,Y) represent the coordinates of a given pixel of an
input image, (X1,Y1) represent the those of the pixel in a dither table,
IN[X][Y] represents an input pixel value, and OUT[X][Y] represents an
output pixel value. The coordinates (X1,Y1) can be rewritten into the
coordinates of a threshold element in the dither matrix that corresponds
to a pixel at the coordinates (X,Y). X_MAX represents the width of the
input image in the main scanning direction, and Y_MAX represents the
width of the input image in the sub-scanning direction. X DMAX represents
the width of the dither table in the main scanning direction, and Y_DMAX
represents the width of the dither table in the sub-scanning direction.
T[X1] [Y1] represents an element in the dither table, and OFFSET[X]
represents a phase offset table. In general, the origin of the
coordinates of a pixel is set to the upper left corner, and is
represented by coordinates (0,0). However, in the flowcharts of FIG. 13
and the like in the embodiment, the origin is set to (1,1), and the
diagonal point is set to (X_MAX,Y_MAX). This is not essential, and is
merely an example.

[0092]In S2 of FIG. 13, the phase offset table OFFSET[X] is created by
referring to the profile characteristic. This table depends on the
X-coordinate of a pixel in the main scanning direction that is obtained
from the profile characteristic. The phase offset table represents an
offset by which the phase of the dither pattern is offset in a direction
opposite to the scan line changing process. FIG. 9D shows an example of
the phase offset table. The phase offset table sets values to return the
dither matrix to an original shape by the scan line changing process.
Assume that the line of interest changes to a line immediately below it
in the sub-scanning direction at the above-mentioned scan line changing
point Pa. In this case, the matrix is offset in advance in a direction
opposite to that of the scan line changing process so as to return the
dither matrix to an original shape by the scan line changing process. In
this example, the direction of the scan line changing process is
downward, this direction is represented by -1, and thus OFFSET[Pa]=1
having an opposite sign. Then, a variable Y is initialized in S3, and
incremented to the next line in S4. In S5, it is determined whether the
position of the pixel of interest has exceeded the sub-scanning width. If
the position of the pixel of interest has exceeded the sub-scanning
width, the process of one page ends. If the position of the pixel of
interest has not exceeded the sub-scanning width, X is initialized in S6,
and incremented to the next digit in S7. In S8, it is determined whether
the position of the pixel of interest has exceeded the main scanning
width. If the position of the pixel of interest has not exceeded the main
scanning width, a pixel represented by the coordinates (X,Y) is set as
the pixel of interest, and the processes in S9 and subsequent steps is
done.

[0093]In S9, an offset OFFSET[X] obtained from the phase offset table in
correspondence with the position X is added to a counter Y. Remainder
calculation is executed for the resultant value using the size of the
dither matrix as a modulus. A dither table sub-scanning counter indicates
the coordinates of a dither matrix element in the sub-scanning direction.
Also in S10, remainder calculation is similarly executed. Note that no
phase need be offset in the main scanning direction. Since dither tables
are periodically arrayed, as shown in FIG. 15, X1 and Y1 are obtained
from the remainders having X-DMAX and Y DMAX as moduli. That is, the
coordinates (X1,Y1) of a threshold element in the dither matrix that
corresponds to a pixel at the coordinates (X,Y) are given by

Y1=(Y+OFFSET[X]) MOD Y--DMAX (2)

X1=X MOD X--DMAX (3)

[0094]From equations (2) and (3), coordinates in a phase-offset dither
table can be obtained.

[0095]In S11, the dither table considering the phase offset amount is
looked up, and the output pixel value OUT is given by

OUT[Y][X]=T[Y1][X1][IN[Y][X]] (4)

[0096]Equation (4) expresses the quantization process. For example,
equation (4) represents a process to compare the threshold T[Y1][X1] with
the input pixel IN[X][Y], and give 1 as OUT[Y][X] if the input pixel
value is larger as a result of comparison, and 0 if the input pixel value
is smaller. By the processes in S9 to S11, the output value of the screen
process considering the amount of phase offset can be attained. The
processes in S4 to S8 are repeated for all pixels in the input image.

[0097]FIGS. 16A to 16G are views schematically showing an intermediate
image and output result in a case where an image process according to the
first embodiment is performed for an input image, and those in a case
where it is not performed. FIG. 16A shows a phase offset table 1601, and
a uniform halftone image 1602 input to the halftone processing unit 407.
FIGS. 16B, 16C, and 16D show a case where no phase offset process
according to the first embodiment is applied. FIGS. 16E, 16F, and 16G
show a case where the first embodiment is applied. FIG. 16B shows an
image 1611 obtained by executing the screen process for the image 1602.
FIG. 16C shows an image 1612 obtained by executing the scan line changing
process for the image 1611. FIG. 16D shows an output result 1613 of the
image 1612. The screen pattern is disturbed by the scan line changing
process.

[0098]To the contrary, FIG. 16E shows an image 1621 obtained by executing
the screen process including the phase offset process for the image 1602
in FIG. 16A. In the image 1621, the screen pattern shifts in a direction
opposite to that of the scan line changing process at the scan line
changing point. FIG. 16F shows an image 1622 obtained by executing the
scan line changing process for the image 1621 in FIG. 16E. By the scan
line changing process, the shift of the screen pattern is canceled and
returns to an original pattern. FIG. 16G shows an output result 1623 of
the image 1622 in FIG. 16F.

[0099]By adding the phase offset process, a mismatch as shown in FIG. 16D
can be eliminated, and an image as shown in FIG. 16G is output to the
storage unit 408. For example, the pulse width of a laser beam is
modulated in accordance with dot image data read out from the second
storage unit 408. A latent image is formed on the photosensitive body in
accordance with the dot image data, and developed with toner. The image
forming unit of each color component executes the registration error
correction process including the scan line changing process, canceling
the registration error of an image formed by the image forming unit of
each color component.

[0100]The first embodiment adds the phase offset process to offset the
phase of the dither matrix in an opposite direction in advance by the
halftone processing unit 407 when reproducing a halftone image by the
screen process. The phase offset process can prevent the phenomenon that
the phase of the dither pattern offsets in the sub-scanning direction
upon the scan line changing process in the storage unit 408. The first
embodiment has described a screen process having a square dither matrix,
but is also applicable to a screen process having a rectangular dither
matrix.

Second Embodiment

[0101]The first embodiment is effective when the dither matrix has a shape
and array as shown in FIGS. 11D and 12. However, the first embodiment
cannot be applied to the array of dither matrices shifted in the main
scanning direction, as shown in FIG. 17A, or a dither matrix of a shape
other than the square or rectangular shape, as shown in FIG. 18A. An
embodiment applicable to even a screen process using a dither matrix
having such a shape and array will be described.

[0102]In the second embodiment, unlike the first embodiment, a halftone
processing unit 407 does not look up a dither table of thresholds stored
in the dither matrix. Instead, the second dither matrix defined by the
shape and array of a dither matrix is generated as a new dither matrix,
and the table (second dither table) of the second dither matrix is looked
up. In the second embodiment, for descriptive convenience, an original
dither matrix will be called the first dither matrix, the dither table of
the first dither matrix will be called the first dither table. Since the
second dither matrix has a simple rectangular shape, the dither matrix
has a shape which can be repetitively applied and can also cover entire
image data by shifting the dither matrix by the matrix size in the
longitudinal and lateral directions.

[0103]FIG. 19 is a flowchart of a screen process including a phase offset
process in the halftone processing unit 407 in the second embodiment. X
and Y represent counters for an image in the main scanning direction and
sub-scanning direction. X2 and Y2 represent counters for the second
dither table in the main scanning direction and sub-scanning direction.
IN[X][Y] represents an input pixel value, and OUT[X][Y] represents an
output pixel value. T' [X2] [Y2] represents the second dither table, and
OFFSET[X] represents a phase offset table. X_MAX represents the width of
an input image in the main scanning direction, and Y_MAX represents the
width of the input image in the sub-scanning direction. X_DMAX represents
the width of the first dither table in the main scanning direction, and
Y_DMAX represents the width of the first dither table in the sub-scanning
direction. X_D2MAX represents the width of the second dither table in the
main scanning direction, and Y D2MAX represents the width of the second
dither table in the sub-scanning direction. The sequence in FIG. 19 is
different from that in FIG. 13 in that the second dither matrix (dither
table) is generated in step S'0 and the second dither matrix is used in
steps S'9 to S'11.

[0104]In S'2, the phase offset table OFFSET is created by referring to the
profile characteristic. In S'0, the second dither table T' is created.
The second dither table T' is a table which contains the first dither
matrix, and holds terms in a rectangular matrix (second dither matrix)
having periodicity within the table. For example, when the dither matrix
has a shape and array as shown in FIG. 17A, a matrix 1701 in FIG. 17B is
generated. When the dither matrix has a shape and array as shown in FIG.
18A, a matrix 1801 in FIG. 18B is obtained as the second dither matrix.
The second dither matrix is not uniquely determined, but suffices to
satisfy the above-described requirement. A generally used dither matrix
is determined in advance, so the second matrix can also be determined in
advance. In this case, in step S'0, the second dither matrix need not be
created and is only referred to. To generate the second dither matrix,
the periods of the first dither matrix in the main scanning and
sub-scanning directions are determined. A matrix having these periods as
longitudinal and lateral sizes is extracted from a threshold table in
which the first dither matrices are arranged without any interval,
obtaining the second dither matrix.

[0105]The threshold table stored in the second dither matrix is attained
as the second dither table T'. In S'9, the second dither table
sub-scanning counter is incremented, and remainder calculation is
executed. In S'11, the output pixel value OUT is determined by looking up
the second dither table T' considering the phase offset obtained from the
attained phase offset table OFFSET. The processes in S'9 to S'11 are
repeated for all pixels in the input image in S'4 to S'8.

[0106]As described above, the second embodiment generates the second
dither matrix, and looks up the second dither table obtained from it. The
second embodiment can perform the phase offset process even in a screen
process using an array of dither matrices shifted in the main scanning
direction or a dither matrix of a shape other than the rectangle.

Third Embodiment

[0107]The third embodiment will exemplify a process when rotating and
printing an image after the screen process and scan line changing
process. FIGS. 20A and 20B are views schematically showing an output
image which is not rotated in an image forming apparatus, and an output
image which is rotated. In FIG. 20A, a line 2001 represents scan line
changing points at which respective scan lines are shifted downward by
the scan line changing process before rotating the image. A line 2002
represents points at which occurrence of registration errors is predicted
when printing after rotating the image. FIG. 20B shows an image after
rotating clockwise through the image shown in FIG. 20A. Lines 2003 and
2004 correspond to lines 2002 and 2001, respectively. Assume that an
electrophotographic image forming apparatus which rotates an input image
after the screen process performs the screen process including the phase
offset process, and the scan line changing process. In this case, if the
first and second embodiments are directly applied, the scan line changing
point and scan line changing direction are not suited to rotated image
data, and no preferable effect can be obtained. More specifically, even
if the scan line changing process and phase offset process are done at
the scan line changing points 2001, scan line changing points 2004
appears along the main scanning direction owing to a 90° rotation
process, as shown in FIG. 20B. The primary purpose of registration error
correction cannot be achieved.

[0108]To prevent this, the third embodiment executes the scan line
changing process and the screen process including the phase offset
process at scan line changing points 2002 after rotation in FIG. 20A on
the premise of the rotation process so as to correct the registration
error when printing a rotated image. An embodiment which considers a scan
line changing point and scan line changing direction after rotation and
is applicable to even a case where an input image is rotated clockwise
through 90°, 180°, and 270° will be explained using
equations. X_MAX and Y_MAX represent widths of an input image in the main
scanning and sub-scanning directions, and X_DMAX and Y_DMAX represent
widths of a dither table in the main scanning and sub-scanning
directions. In this phase offset processing system, (X,Y) represent the
coordinates of a given pixel, IN[X][Y] represents the pixel value,
Xo_OFFSET[Y] represents a phase offset table in the main scanning
direction when no image is rotated, and Yo_OFFSET[X] represents a phase
offset table in the sub-scanning direction. (Xn,Yn) represent the
coordinates of the pixel in the coordinate system of a rotated image when
an input image is rotated clockwise through an angle of n. Xr_OFFSET[Xn]
[Yn] [n] represents a phase offset table in the main scanning direction,
and Yr_OFFSET[Xn][Yn][n] represents a phase offset table in the
sub-scanning direction. The suffix "n" represents the rotational angle.

[0109]The phase offset table Xo_OFFSET[Y] in the main scanning direction
when no image is rotated is always constant at 0 regardless of Y. As
shown in FIG. 21, the coordinates (Xn,Yn) and (X,Y) in the coordinate
system after rotation satisfy relations given by equations (5) to (8):

X=Y90=X_MAX-X180=X_MAX-Y270 (5)

Y_MAX-Y=X90=Y180=Y_MAX-X270 (6)

X_MAX-X=X_MAX-Y90=X180=Y270 (7)

Y=Y_MAX-X90=Y_MAX-Y180=X270 (8)

[0110]Based on these equations, main scanning and sub-scanning phase
offset amounts at the respective rotational angles are given by

##EQU00001##

[0111]Since the second dither tables are periodically arrayed, X1 and Y1
can be obtained from the remainders of X_D2MAX and Y_D2MAX, deriving
equations (15) and (16), wherein X1 and Y1 are the coordinates of an
element in the first dither table:

Y2=(Y+Xr_OFFSET[X][Y][n]) MOD Y_D2MAX (15)

X2=(X+Yr_OFFSET[X][Y][n]) MOD X_D2MAX (16)

[0112]The pixel value of an output image is given by

OUT[Y][X]=T'[Y2][X2][IN[Y][X]] (17)

where T'[Y2][X2] is the second dither table.

[0113]From equations (9) to (17), the output value of a screen process
considering the amount of phase offset after rotation can be attained.
Even an image forming apparatus which performs the rotation process after
the halftone process can execute the phase offset process.

Fourth Embodiment

[0114]FIGS. 22A to 22C are views schematically showing an unrotated output
image, a rotated output image, and an intermediate image when rotating an
output image in the fourth embodiment. The third embodiment has
exemplified a phase offset process in an image forming apparatus which
performs the rotation process after the halftone process. In this case,
the array angle (to be referred to as a screen angle hereinafter) of the
dither pattern is different between an output image not rotated after the
screen process and a rotated output image, as shown in FIGS. 20A and 20B.
Owing to the engine characteristic of the image forming apparatus, the
gamma value of halftoning changes between the case where an output image
is rotated and the case where no output image is rotated, losing the
isotropy of an output image. To solve this problem, the dither matrix is
rotated counterclockwise through the same angle (rotational angle) as
that of an image, as shown in FIG. 22B, when performing the screen
process for an image shown in FIG. 22A (this process will be referred to
as the first rotation process hereinafter). Then, the screen angle
returns to the original one after rotation (to be referred to as the
second rotation process hereinafter), obtaining a preferable output image
as shown in FIG. 22C. A line 2201 in FIG. 22A represents scan line
changing points at which respective scan lines are shifted downward by
the scan line changing process before rotating the image. A line 2202 in
FIG. 22C represents points at which occurrence of registration errors is
predicted when printing after rotating the image. A line 2203 in FIG. 20B
represents a scan line changing points at which respective scan lines are
occurred shifted downward by the scan line changing process in the
rotated image.

[0115]An embodiment applicable to even a case where an electrophotographic
image forming apparatus having this function rotates an input image
clockwise through 90°, 180°, and 270° will be
described.

[0116]The fourth embodiment is different from the third embodiment in that
the first rotation process is done to rotate the dither matrix
counterclockwise (i.e., in a direction opposite to the rotational
direction of image data) through an angle of n. (X1n,Y1n) represent the
coordinates of a pixel in the dither table when the first rotation
process is performed to rotate an input image clockwise through an angle
of n. Tr[Y1n] [X1n] [n] represents a dither table in the coordinate
system when the dither table is rotated counterclockwise at an angle of
n. X_DMAXn represents the width of the dither table in the main scanning
direction, and Y_DMAXn represents the width of the dither table in the
sub-scanning direction.

[0117]As shown in FIG. 23, the coordinates (X,Y) and the coordinates
(Xn,Yn) in the coordinate system after counterclockwise rotation satisfy
relations given by equations (17) to (20):

X1=X--DMAX-Y190=X--DMAX-X1180=Y1270 (17)

Y--DMAX-Y1=Y--DMAX-X190=Y1180=X1270 (18)

X--DMAX-X=Y190=X1180=X1--DMAX-Y1270 (19)

Y1=X190=Y--DMAX-Y1180=Y--DMAX-X1270 (20)

[0118]Since the lengths of the respective sides of the dither matrix are
equal to each other, as shown in FIG. 23, equations (21) and (22) are
established:

X--DMAX=Y--DMAX90=X--DMAX180=Y--DMAX270 (21)

Y--DMAX=X--DMAX90=Y--DMAX180=X--DMAX270 (22)

[0119]From equations (17) to (22), the dither table Tr, X1n, Y1n, X_DMAXn,
and Y_DMAXn are obtained. As a result, the same conditions as those in
the third embodiment are given, and subsequent calculation of the pixel
value of an output image complies with the third embodiment.

[0120]By setting in advance the screen angle by the same amount as
rotation of an image in an opposite direction, the screen angle returns
to an original one upon rotation of image data. A preferred image is
formed without changing the gamma value of the halftone process.

Other Embodiments

[0121]In the above-described embodiments, the screen process and
registration error correction process are done by rotating image data or
the dither matrix. However, it is also possible to perform
horizontal/vertical conversion for an address from which the pixel of
image data or the element of a matrix is read out as if rotated data were
referred to. Even in this case, the term "rotation process" is valid
because this process is substantially the same as rotation of image data
or the dither matrix.

[0122]The present invention may also be applied to a system including a
plurality of devices (e.g., a host computer, interface device, reader,
and printer), or an apparatus (e.g., a copying machine or facsimile
apparatus) formed by a single device. The object of the present invention
is also achieved by supplying a storage medium which stores program codes
for implementing the functions of the above-described embodiments to a
system or apparatus, and reading out and executing the program codes
stored in the storage medium by the computer of the system or apparatus.
In this case, the program codes read out from the storage medium
implement the functions of the above-described embodiments, and the
storage medium which stores the program codes constitutes the present
invention.

[0123]The present invention also includes a case where an OS (Operating
System) or the like running on the computer performs part or all of
actual processing based on the instructions of the program codes and
thereby implements the functions of the above-described embodiments.
Further, the present invention is also applied to a case where the
program codes read out from the storage medium are written in the memory
of a function expansion card inserted into the computer or the memory of
a function expansion unit connected to the computer. In this case, the
CPU or the like of the function expansion card or function expansion unit
performs part or all of actual processing based on the instructions of
the written program codes, and thereby implements the functions of the
above-described embodiments.

[0124]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0125]This application claims the benefit of Japanese Patent Application
No. 2007-199901, filed Jul. 31, 2007 which is hereby incorporated by
reference herein in its entirety.